Caesium (Also spelled Cesium) is a chemical element in the periodic table that has the symbol Cs and atomic number 55. This is a soft silvery-gold Alkali metal which is one of the three metals that are liquid at room temperature. This element is most notably used in atomic clocks.

Caesium is sometimes spelt cesium, especially by Americans, but caesium is the official name preferred by IUPAC, although since 1993 it has recognized cesium as a variant.

Cs-134 has been used in hydrology as a measure of cesium output by the nuclear power industry[?]. This isotope is used because, while it is less prevalent than either Cs-133 or Cs-137, Cs-134 can be produced solely by nuclear reactions. Cs-135 has also been used in this function.

Like other group 1 elements, caesium has a great affinity for oxygen and is used as a "getter" in electron tubes[?].

It can be isolated by electrolysis of fused cyanide and in a number of other ways.
Exceptionally pure and gas-free caesium can be made by the thermal decomposition of caesium azide.
The primary compounds of caesium are its chloride and its nitrate. The price of cesium in 1997 was about $US 30 per gram.
Isotopes
Caesium has 32 known istotopes which is more than any other element. The atomic masses of these isotopes range from 114 to 145.
Even though this element has the largest number of isotopes, it only has one naturally occurring stable isotope, Cs-133. The radiogenic isotope Cs-137 has been used in hydrologic studies, analogous to the use of H-3. Cs-137 is produced from detonation of nuclear weapons and emissions from nuclear power plants. Beginning in 1954 with the commencement of nuclear testing, Cs-137 was released into the atmosphere where it is absorbed readily into solution. Once Cs-137 enters the ground water, it is deposited on soil surfaces and removed from the landscape primarily by particle transport[?]. As a result, the input function of these isotopes can be estimated as a function of time.
Precautions Caesium is highly explosive in cold water. Caesium should be considered highly toxic. Some of its
radioisotopes are even more toxic.